109 lines
4.5 KiB
Markdown
109 lines
4.5 KiB
Markdown
# `<nixpkgs/nixos/lib/make-disk-image.nix>` {#sec-make-disk-image}
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`<nixpkgs/nixos/lib/make-disk-image.nix>` is a function to create _disk images_ in multiple formats: raw, QCOW2 (QEMU), QCOW2-Compressed (compressed version), VDI (VirtualBox), VPC (VirtualPC).
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This function can create images in two ways:
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- using `cptofs` without any virtual machine to create a Nix store disk image,
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- using a virtual machine to create a full NixOS installation.
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When testing early-boot or lifecycle parts of NixOS such as a bootloader or multiple generations, it is necessary to opt for a full NixOS system installation.
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Whereas for many web servers, applications, it is possible to work with a Nix store only disk image and is faster to build.
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NixOS tests also use this function when preparing the VM. The `cptofs` method is used when `virtualisation.useBootLoader` is false (the default). Otherwise the second method is used.
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## Features {#sec-make-disk-image-features}
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For reference, read the function signature source code for documentation on arguments: <https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/make-disk-image.nix>.
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Features are separated in various sections depending on if you opt for a Nix-store only image or a full NixOS image.
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### Common {#sec-make-disk-image-features-common}
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- arbitrary NixOS configuration
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- automatic or bound disk size: `diskSize` parameter, `additionalSpace` can be set when `diskSize` is `auto` to add a constant of disk space
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- multiple partition table layouts: EFI, legacy, legacy + GPT, hybrid, none through `partitionTableType` parameter
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- OVMF or EFI firmwares and variables templates can be customized
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- root filesystem `fsType` can be customized to whatever `mkfs.${fsType}` exist during operations
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- root filesystem label can be customized, defaults to `nix-store` if it's a Nix store image, otherwise `nixpkgs/nixos`
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- arbitrary code can be executed after disk image was produced with `postVM`
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- the current nixpkgs can be realized as a channel in the disk image, which will change the hash of the image when the sources are updated
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- additional store paths can be provided through `additionalPaths`
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### Full NixOS image {#sec-make-disk-image-features-full-image}
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- arbitrary contents with permissions can be placed in the target filesystem using `contents`
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- a `/etc/nixpkgs/nixos/configuration.nix` can be provided through `configFile`
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- bootloaders are supported
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- EFI variables can be mutated during image production and the result is exposed in `$out`
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- boot partition size when partition table is `efi` or `hybrid`
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### On bit-to-bit reproducibility {#sec-make-disk-image-features-reproducibility}
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Images are **NOT** deterministic, please do not hesitate to try to fix this, source of determinisms are (not exhaustive) :
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- bootloader installation have timestamps
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- SQLite Nix store database contain registration times
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- `/etc/shadow` is in a non-deterministic order
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A `deterministic` flag is available for best efforts determinism.
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## Usage {#sec-make-disk-image-usage}
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To produce a Nix-store only image:
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```nix
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let
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pkgs = import <nixpkgs> {};
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lib = pkgs.lib;
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make-disk-image = import <nixpkgs/nixos/lib/make-disk-image.nix>;
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in
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make-disk-image {
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inherit pkgs lib;
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config = {};
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additionalPaths = [ ];
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format = "qcow2";
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onlyNixStore = true;
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partitionTableType = "none";
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installBootLoader = false;
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touchEFIVars = false;
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diskSize = "auto";
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additionalSpace = "0M"; # Defaults to 512M.
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copyChannel = false;
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}
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```
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Some arguments can be left out, they are shown explicitly for the sake of the example.
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Building this derivation will provide a QCOW2 disk image containing only the Nix store and its registration information.
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To produce a NixOS installation image disk with UEFI and bootloader installed:
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```nix
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let
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pkgs = import <nixpkgs> {};
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lib = pkgs.lib;
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make-disk-image = import <nixpkgs/nixos/lib/make-disk-image.nix>;
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evalConfig = import <nixpkgs/nixos/lib/eval-config.nix>;
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in
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make-disk-image {
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inherit pkgs lib;
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config = evalConfig {
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modules = [
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{
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fileSystems."/" = { device = "/dev/vda"; fsType = "ext4"; autoFormat = true; };
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boot.grub.device = "/dev/vda";
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}
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];
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};
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format = "qcow2";
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onlyNixStore = false;
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partitionTableType = "legacy+gpt";
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installBootLoader = true;
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touchEFIVars = true;
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diskSize = "auto";
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additionalSpace = "0M"; # Defaults to 512M.
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copyChannel = false;
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memSize = 2048; # Qemu VM memory size in megabytes. Defaults to 1024M.
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}
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```
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